Machining, such as turning, drilling or milling, frequently causes problems of vibrations and flexions, particularly when the length of the non-clamped or canti-lever part of the tool exceeds 4-5 times the diameter.
Vibrations and flexions are closely related. When applying the tooth to the workpiece there will first be a medium flexion caused by the cutting force applied to the tooth. When the tool is vibrating, a reciprocating motion will occur above and below the medium flexion (see FIG. 1). This motion will be amplified if the frequencies applied by the cutting force approaches the resonance frequencies of the tool.
Vibration problems usually occur in internal cutting, when deep cutting is intended and the possibility of increasing the diameter of the tool holder is low. Two types of vibrations are particularly problematic; flexional vibrations and torsional vibrations. These vibrations result in poor productivity, poor surface finish, reduced tool working life, and they often prevent machining.
Because of tool flexion, the intended dimension or the intended shape will not be obtained upon a cutting operation. By permitting adjustments of the medium flexion, one could come closer to the intended dimension or shape. One could also control the medium flexion to achieve a shape of the end product that otherwise would require a special tool.
Workpieces, particularly workpieces having a thin-walled cross section, are also subject to the same problems of medium flexion and vibrations. It is then usually the workpiece that flexes, while the tool is more at rest.
Vibration damping for machining has hitherto usually been performed by using passive mechanical dampers in which a mass of heavier material is supported in spring and damper elements (twin mass system) which in turn are supported in the tool (see U.S. Pat. No. 5,413,318, for example). The problem associated with mechanical dampers is i.a. that the heavy materials are expensive, each tool is limited to specific cantilever lengths and that the damper occupies space in the tool holder, thus weakening the tool. The materials oil and rubber are frequently used and they may be hard to obtain in a stable quality and they change properties with temperature and working life. In addition, such damping systems have limitations as to how low frequencies may be achieved. Also, twin mass dampers add an additional mass, hampering the balancing of tools rotating at a higher rpm.
Active dampening of tool holders may be achieved, for example, by using piezoelectric force actuators. Such force actuators have previously been used i.a. in passive electrical dampers, such as in shunted force actuators in skis, tennis rackets and golf clubs. In active systems typically a piezoelectric force actuator is used which is bonded or otherwise attached to or within the tool holder. The actuator will then transmit the force to the tool via shear forces. A control system, typically an adaptive regulating system, controls the actuator force by means of information from a sensor, typically an accelerometer. In order to be able to damp vibrations in such a tool in the best possible way, the actuator has to be located close to the tool holder clamp. The problem associated with the said locations of actuators is the fact that they do not allow flexibility along the length of the overhang. Also, the force transmission to the tool will be inefficient since these shear forces have to be very large in order to resist motions farthest out on the tooth tip.
The prior art comprises positioning of actuators directly onto or recessed in pockets on the tool holder, and the forces will then be transmitted from the actuator to the tool holder via shear forces. With such a clamping of actuators, one will be locked with respect to overhang lengths and force direction.